About 350 new U.S. troops will begin arriving in Iraq on Wednesday, bringing the total American military footprint there above 1,000 personnel for the first time in nearly three years.

President Obama approved the additional troops — mostly soldiers and Marines — late Tuesday night. They will arrive via U.S. military aircraft and be assigned primarily to diplomatic security duties around the U.S. Embassy in Baghdad, military officials said.

The latest deployment reflects a growing concern about the general dangers posed by Islamic State militants who have seized large swaths of territory in Iraq and Syria and continue to threaten neighborhoods on the outskirts of Baghdad.

“There is no specific threat that has triggered this,” Army Col. Steve Warren, a Pentagon spokesman, said Wednesday.

About 861 U.S. troops are in Iraq, and the new deployments will bring that total to more than 1,200, Warren said. That will mark the highest American force levels since the withdrawal of U.S. combat forces in December 2011.

A small detachment of H-60 helicopters is on the ground in Iraq, helping to move U.S. personnel around the country and standing by prepared for medevac missions if needed, Warren said.

A total of about 400 U.S troops from elsewhere in U.S. Central Command will arrive in Iraq over the next several days, and about 50 are leaving and returning to CENTCOM locations outside Iraq, resulting in a net increase of about 350 in Iraq, Warren said.

Obama, traveling in Estonia on Wednesday, responded to critics who say the U.S. is moving slowly in responding to the IS militants.

“It’s going to take time for us to be able to roll them back. And it is going to take time for us to be able to form the regional coalition that’s going to be required so that we can reach out to Sunni tribes in some of the areas that [the Islamic State] has occupied, and make sure that we have allies on the ground in combination with the airstrikes that we’ve already conducted,” Obama said.

“So the bottom line is this: Our objective is clear, and that is to degrade and destroy [the IS force] so that it’s no longer a threat not just to Iraq but also the region and to the United States,” Obama said.

Meanwhile, U.S. airstrikes continued to pound Islamic State militia targets in northern Iraq, where a fight for control over the Mosul Dam continues. Iraqi security forces, with the help of U.S. airstrikes, seized control of the dam from militants Aug. 18, but IS forces continue to mount counterattacks.

“ISIS keeps trying to take it back,” Rear. Adm. John Kirby, a Pentagon spokesman, said Tuesday, referring to the dam.

“As long as they continue to pose a threat to the facility, we are going to continue to hit them,” Kirby said.

Obama has signaled a reluctance to expand U.S. airstrikes into other regions of Iraq or Syria where IS forces also hold territory.

“It is very important from my perspective that when we send our pilots in to do a job, that we know that this is a mission that’s going to work, that we’re very clear on what our objectives are, what our targets are,” Obama said Wednesday morning.